Septoria and Stagonospora Diseases of Cereals - CIMMYT ...

96
Session 5 — M.W. Shaw
The pathogen appears to have
been becoming more widespread
worldwide. A most interesting
series of data is provided by Gilbert
(1998) showing a more or less
linear increase in M. graminicola
incidence between 1990 and 1995 in
southern Manitoba, from a very
scarce disease to the most
prevalent. Coupled with the
genetic evidence that populations
worldwide are genetically very
similar (McDonald et al., 1995) it
does seem possible that a novel
form of the pathogen has been
spreading worldwide. If true, this
would raise the interesting
question as to what
epidemiological characteristic
confers the new form’s
invasiveness.
Phaeosphaeria
nodorum
Within crops
Multiplication is more rain
dependent than M. graminicola,
since pycnidia are produced in
response to rain as well as
dispersed by it. Conidia require
wet periods to infect and do not
tolerate interruptions well. The
pathogen also appears to multiply
ineffectively in cold conditions.
Correlations with weather
conditions are much better (Djurle
et al., 1996; Gilbert et al., 1998) and
more closely linked in time to
serious outbreaks than for M.
graminicola. In wet, warm weather,
typical of summer in some regions,
latent periods can be shorter than
in M. graminicola. Progress up the
growing plant is correspondingly
faster and more regular
(Mittermeier and Hoffmann, 1984).
Like M. graminicola spores,
spores of some isolates of P.
nodorum infect with a startlingly
high probability when rare on a leaf
surface, the infection efficiency per
spore falling rapidly with total
numbers attacking in a droplet
(Jeger et al., 1985)(MWS,
unpublished). The ecological
advantage is presumably that
reinfection of a new crop is very
effective, after which survival of the
isolate is more or less certain. The
epidemiological consequence is
surprisingly rapid early spread of
the pathogen with subsequent rapid
decline in apparent epidemic rate.
Between crops
Phaeosphaeria nodorum can be
seedborne, and before routine
fungicide treatment of seed and the
upper parts of the crop, substantial
numbers of seeds were infected. In
some regions, such as northern
Europe, the disease appears to have
become much less important during
the 1980s and 1990s. It is tempting
to ascribe this decline to the
increased use of fungicides
reducing seed infection both in the
ear and at planting. Recent
experiments in other parts of the
world suggest that seedborne
inoculum is crucial in many places
(Milus and Chalkley, 1997).
Alternate hosts seem to harbor
isolates of the pathogen capable of
attacking wheat, but there is
sufficient partial specialization that
these do not act as the source of
most infections on wheat as well as
isolates clearly partially specialized
to other hosts (Krupinsky, 1997a;
1997b). Crop residues have been
shown to be important (Cunfer,
1998) but international survey
evidence does not suggest they are
the dominant source of inoculum
(Leath et al., 1994).
Ascospores have been found,
but the evidence for their ubiquity
and epidemiological importance
seems to vary geographically.
Trapping experiments like those
used to show how airborne
inoculum of P. nodorum varies
through the year have been
published by Cunfer (1998). These
showed that in Georgia, USA,
inoculum persisted for long periods
in stubble, detectable sporadically
for at least 18 months after the crop
was harvested. As little as 17 m
from a field edge, inoculum was
trapped only once, in March, even
though both mating types of the
fungus were present and could
potentially form perithecia. This
suggests that sexual reproduction is
unusual, though possibly not
absent.
These data are consistent with
the relatively old published data on
ascospore release in France (Rapilly
et al., 1973). Anecdotally, near
Reading in 1994 we observed a
sudden patch of the disease in May
in a first wheat. Although all
isolations were from a single plot,
and the disease had not been
previously observed in that crop, 13
genotypes tested using RFLPs were
of different genotypes (C.N.F.M.R.J
Pijls and M.W. Shaw, unpublished).
However, the most extensive
study made, in Poland,
demonstrated a quite different
picture, with ascospores of P.
nodorum present throughout the
year and at densities that appeared
to be independent of distance from
the nearest infected plot (Arseniuk
et al., 1998). Ascospores were also
found throughout the year in
northern Germany, but the trap was
operated over an unharvested field,
so it is not possible to say whether